turning spindle speed

[Mutt]

RPM = SFM x 4 ÷ diameter

Tom

Thanks Tom ,I'll write this down

 

[pontiac428]

If you want a simple formula, at least use one that is accurate.

SFM=(11/42)*diameter*RPM

RPM=(SFM*(42/11))/diameter

That will get you to 0.5% of your target. It makes no sense to rely on a formula that is less than 20% accurate even without the typo, especially when it makes the difference between a sheave or two too fast when picking your turning speed. We have all this expensive gear for measuring tenth-thousandths and micrometers, so why slop around with kludgy math in this case?

 

[higgite]

If you want a simple formula, at least use one that is accurate.

SFM=(11/42)*diameter*RPM

RPM=(SFM*(11/42))/diameter

That will get you to 0.5% of your target. It makes no sense to rely on a formula that is less than 20% accurate even without the typo, especially when it makes the difference between a sheave or two too fast when picking your turning speed. We have all this expensive gear for measuring tenth-thousandths and micrometers, so why slop around with kludgy math in this case?

Your first sentence makes a valid point for those who require pinpoint accuracy. But you better recheck your algebra. Your RPM formula RPM=(SFM*(11/42))/diameter yields RPM = SFM X .262 ÷ diameter.

The exact formula is in fact RPM = SFM x 12 ÷ (π x diameter in inches), which yields RPM = SFM x 3.8197 ÷ diameter. Using 4 instead of 3.8197 introduces a 4.7% error in an estimated speed that is calculated using SFM that one arbitrarily chooses from a suggested range of SFM. If that is critical to your operation, then please do the math with 3.8197 instead of 4. But, please don’t condescend to us who choose to use the number 4 for convenience.

Tom

 

[P. Waller]

sousing this formula and the littlemachineshop chart a piece of 12L14, 1.375" diameter spun at 750 rpm would be 270 SFM ?

I think what I am more interested in is what rpm various types of metals need to be turned at. So if I know the SFM and the diameter of the work piece, what is the formula?

I gave that earlier, I round off to 2 decimal places.
1.37" diameter = 4.32" Circumference (1.37 X pi)
4.32" / 12 is .35 Feet Circumference
This will require the part to pass under the tool at 1 / .351 = 2.85 Revolutions Per Foot
270 Feet Per Minute cut speed therefore requires a rotational speed 2.85 times higher than the Cut Speed, 270 X 2.85 = 760 Revolutions Per Minute.

Or simply buy a calculator for this, I like the Machinists Calc Pro app, I do realize that $29.99 is considered expensive here. I find it is well worth $30 however.
https://play.google.com/store/apps/details?id=com.calculated.louise&hl=en_US